Enhanced performance of microbolometer using coupled feed horn antenna Kuntae Kim*,a, Jong-Yeon Park*, Ho-Kwan Kang*, Jong-oh Park*, Sung Moon*, Jung-ho Park a * Korea Institute of Science and Technology, Seoul, Korea a Korea University, Seoul, Korea ABSTRACT In the paper, we improved the performance of the microbolometer using coupled feed horn antenna. The response time of the device was improved by reducing thermal time constant as the area of the absorption layer was reduced. We designed the shape of an absorption layer as circular structure in order to reduce the coupling loss between the antenna and the bolometer. A supporting leg for thermal isolation also has circular structure and its length increased up to 82 μm, it reduced the thermal conductance to 4.65 10-8 [W/K]. The directivity of the designed antenna has 20.8dB. So the detectivity of the bolometer was improved to 2.37 10-9 [ cm Hz / W ] as the noise characteristics of the bolometer was enhanced by coupling feed horn antenna. The fabrication of the bolometer are carried out by a surface micromachining method that uses a polyimide as a sacrificial layer. The absorption layer material of the bolometer is VO x and its TCR value has above 2%/K. The 3-D feed horn antenna structure can be constructed by using a PMER negative photoresist. The antenna and the bolometer can be bonded by Au-Au flip chip bonding method. Keywords: microbolometer, feed horn antenna, antenna coupled, detectivity 1. INTRODUCTION Recently, the demand for inexpensive and uncooled infrared detectors has grown for both civilian and military application. In this respect, thermal detector has advantage over photon detector. 1. Thermal detector can be divided into three types: 1)bolometer, 2)thermopile, 3)pyroelectric detector. 2. The bolometer is most widely researching now because it doesn t need the chopper and it can be fabricated monolithically. 3,4,5,6,7,8. The bolometer converts absorbed IR radiation into heat, which in turn changes the resistance of the absorption layer. The bolometer can be modeled as an IR-sensitive element of thermal mass C linked via a thermal conductance G to a substrate serving as the heat sink. The performance of the bolometer is characterized by certain figures of merit such as temperature coefficient of resistance *Contact: E-mail: korion@kist.re.kr, phone:82-2-958-6809
(TCR), responsivity R and detectivity D*. The TCR is an measure of how rapidly the electrical resistance of a material responds to a change in temperature, the responsivity is the output signal voltage per unit incident infrared power and the detectivity D* is a figure of merit that normalizes the performance of the bolometer with respect to the size and noise signal. To date, bolometer array structures have been fabricated as open structure which has little directionality and, as a consequence, are sensitivity to radiation incident from all angles. The result of this is a degradation of the signal to noise ratio of each array pixel element and a corresponding decrease in the overall detectivity of the bolometer array. A method of enhancing individual pixel directionality is therefore desirable and can be achieved by the use of feed horn antenna. 9. The feed horn would provide efficient coupling between focal plane image and the bolometer array and would significantly reduce the amount of radiation incident on a given pixel element. And it can lead to significant improvement in detectivity of bolometer. Besides of this effect, the size of the bolometer is reduced, so it can lead to reducing the total power consumption and it can be applied to the fast image detection system. Figure 1 is schematic view of the proposed feed horn antenna coupled bolometer. Figure 2 shows the advantage of the antenna coupled bolometer by reducing the signal to noise ratio. Figure 1: Schematic view of feed horn antenna coupled bolometer Figure 2: Advantage of the antenna coupled bolometer
2. DESIGNS We designed the bolometer and the antenna focusing on enhancement of the detectivity. First, we designed optimal size of the antenna that has most excellent directivity in 10 μm wavelength. The directivity of antenna expressed as 10. D ( db) = 10log c 2 10 s 4π 2 C ε ap ( πa) = 10log10 L( ) 2 (1) λ λ 2 3 d L( s) = 10log10( ε ap) (0.8 1.71s + 26.25s 17.79s ), m s = 8λl where a is radius of horn at the aperture, L(s) is directivity loss for aperture efficiency, and C is aperture circumference and s is maximum phase deviation. Figure 3 shows designed feed horn antenna structure and simulation result of the antenna s directivity. Directivity of the designed feed horn antenna has 20.8dB where the radius of horn is 54 μm. Next, we determined the width of an absorption layer and the length of a thermal isolation leg for most excellent detectivity. Absorption layer width of the bolometer was matched to the diameter of the antenna in order to reduce the coupling loss between the bolometer and the antenna. The thermal conductance G and heat capacitance C of the bolometer has 4.65 10-8 [W/K] and 9.31 10-10 [J/K] respectively from the following equations. 11. C = ρcv, where ρ : density, c:heat capacity, V:volume for absorption layer G = K Wd l, where K:thermal conductance, W:width, d:thickness, l:length for thermal isolation leg The detectivity D* was calculated using the above equations and following equation (2) and it s value has 2.37 10 9 [ cm Hz / W ] αriη Ad f D* = 2 2 2 G 1+ ω τ Vn (2) Where α (TCR):0.02K -1, R(bolometer resistance):100k, η (absoption ratio):0.9, I(bias current):5 μa, τ (thermal time constant):0.02s, ω =2π 30Hz, Ad(absoption layer area):π 11 μm 2, 2 Vn f 9 4kTR = 40.7 10. Figure 4 shows the final designed antenna coupled bolometer. 2 54 μm 44 μm 22 μm Figure 3: Designed antenna structure and directivity simulation
54 μm Antenna 22 μm 2.5 μm Substrate Absorption layer Supporting leg Figure 4: Designed antenna coupled bolometer 3. FABRICATIONS 3.1 Antenna fabrication Antenna fabrication can be carried out like figure 5. Firstly, deposit Cr/Au seed layer. Secondly, tilted and rotated illumination on PMER negative photoresist. After developed the photoresist, the horn shape mold are formed. Thirdly, electroplate the Ni on the seed layer using the PMER mold. After chemical mechanical polishing(cmp) process, removing the PR mold and seed layer, then the antenna is constructed. The key technology to make the horn shape mold is tilted and rotated illumination using a Mirror Reflected Parallel Beam Illuminator(MRPBI) which is invented for parallel beam illumination. Figure 6 is MRPBI system and Figure 7 is PMER mold which was made by using the MRPBI system. 1. seed layer deposition 4. CMP 2. Tilted and rotation illumination 5. PR removing 3.Electroplating 6. Seed layer removing Figure 5: Antenna fabrication process
Lamp housing Rear reflector UV cold mirror Shutter, filter Optical board Sample stage Motor, gear, sensor Figure 6: MRPBI system Figure 7: PMER mold made by MRPBI 3.2 Bolometer fabrication The bolometer can be made by surface micromachining method. Figure 8 shows the bolometer fabrication process. Firstly, polyimide was used as a sacrificial layer. Polyimide was cured on high temperature oven and had 2.5 μm thickness, it is etched by RIE using AZ9260 thick PR which plays a role as an etch mask. The polyimide has to rounded side-wall for post SiNx, metal deposition and patterning. Figure 9 shows SEM image of the rounded side-wall of polyimide and 2 4patterned polyimide array. Secondly, a SiNx was used as a thermal isolation supporting leg. SiNx was deposited using the PECVD and it was patterned using the RIE. Thirdly, a VOx was used as a absorption layer and it s TCR has above 2%. Figure 10 is optical image of a patterned SiNx and VOx. Fourthly, Cr/Au layer for contact layer was deposited using the evporator and it was patterned using the RIE. Finally, ashing the sacrificial layer using the microwave plasma asher. Figure 11 is a single pixel mask layout and patterned bolometer. Figure 12 shows 2 4 patterned bolometer array. After fabricated the antenna and the bolometer respectively, they are bonded by Au-Au flip chip bonding like figure 13.
Mask 1 Mask 4 1. Polyimide patterning 4.Cr/Au etching Mask 2 Mask 2 2. Si3N4 etching 5. Si3N4 etching(upper layer) Mask 3 3. VOx etching 6. Sacrifitial layer removing Figure 8: Bolometer fabrication process (a) Figure 9: Patterned polyimide (a)rounded side-wall of polyimide, (b) 2 4 patterned bolometer array (b) (a) Figure 10: Patterned SiNx and VOx (b) (a) single pixel, (b) 2 4 array
Figure 11: Mask layout and patterned bolometer Figure 12: 2 4 bolometer array 54 μm 3 μm 22 μm 1 μm 15 μm Au 5 μm Figure 13: Bonding of the antenna and the bolometer
4. CONCLUSIONS We improved detectivity of the bolometer using the couped feed horn antenna which increases signal to noise ratio. Optimal size of the antenna and the bolometer was designed in order to enhance the detectivity of the bolometer. Antenna simulation was carried out by HFSS simulation tool. The directivity of designed antenna had 20.8dB and the detectivity of the device had 2.37 10-9 [ cm Hz / W ]. To make the horn shape antenna, we invented MRPBI system which can illuminate parallel beam and it s stage can be tilted and rotated. We convinced that the bolometer can be fabricated by surface micromachining method. We also confirmed that the antenna and the bolometer can be fabricated respectively and they can be bonded each other by Au-Au flip chip bonding. ACKNOWLEDGMENTS This work has been supported by the 21C frontier project and Intelligent microsystem program REFERENCES 1. Paul W.Kruse, A comparison of the limits to the performance of thermal and photon detector imaging arrays, Infrared Phys. Techno., 36, pp.869-882, 1995. 2. Djuric, New generation of thermal infrared detectors, International conference on microelectronics, 2, pp.559-564, 1995. 3. Paul W.Kruse, Uncooled Infrared Imaging Arrays and Systems, Semiconductors and Semimetals, vol.47, 1997. 4. E. Cole, "Monolithic Two-Dimensional Arrays of Micromachined Microstructures for Infrared Applications", Proceedings of the IEEE, 86, pp.1679-1686, 1998. 5. Herbert Jerominek, Micromachined, uncooled, VO 2 -based, IR bolometer arrays, SPIE, 2746, pp.60-71, 1996. 6. K.C. Liddiard, Thin film monolithic detector array for uncooled thermal imaging, SPIE, 1969, pp.206-216, 1993. 7. Paul W. Kruse, "Uncooled Infrared Focal Plane Arrays", Proceedings of the IEEE, pp.643-646, 1995. 8. I.A. Khrebtow, "Uncooled thermal IR detector arrays", J.Opt.Technology, 64, pp.511-519, 1997. 9. Gabriel M. Rebeiz, Monolithic millimeter-wave two-dimensional horn imaging arrays, 38, pp.1473-1482, 1990. 10. Constantine A. Balanis, Antenna Theory: analysis and design, John wiley & sons, p.695, 1997. 11. John E. Gray, "MgO Sacrificial Layer for micro-machining Uncooled Y-Ba-Cu-O IR Microbolometers on Si 3 N 4 Bridges, IEEE J. of MEMS, 8, pp.192-199, 1999.